The objective of this paper is the investigation and evaluation of the performance and life expectancy of NPT threaded connections as applied to a particular type of reciprocating compressor units.
The study focused on the assessment of the levels of excitation forces acting on the threaded connections and the resulting stress. These forces and stresses were determined from field vibration measurements results applied to a mechanical model of the system.
As an extensive literature search did not yield satisfactory data for specific fatigue properties of NPT connections, we applied a first-principles approach to the problem and calculated the stress concentration factors from a detailed Finite Element Study of the thread engagement under axial and circumferential loads. The results and details are reported here.
Stress levels determined by this methodology were compared to the allowable fatigue limits for the piping and valves materials and a life expectancy was calculated.
The same methodology can be applied to other situations involving threaded connections.
In the process of evaluation for procurement, end users often have to make a decision between buying equipment that meets the operating conditions for the project but does not exactly conform to some of the user's own specifications or equipment with unacceptably long delivery times or that is even unavailable. Here it is shown a case study that exemplifies how decision makers can evaluate the reliability of equipment and therefore minimize risks while making a decision favoring the first option.
Several given conditions of operation were considered in this study. The stress levels occurring on the threaded joints is strongly governed by the 'stress concentration factors' characteristic of the thread profile. An extensive literature search did not yield satisfactory data, either theoretical or empirical for specific fatigue properties of NPT connections or for the stress concentration at the threads. A first-principles approach to the problem was selected and evaluation of the stress concentration factors from a detailed Finite Element Study of the thread engagement under axial and circumferential loads was undertaken.
The forces acting on the piping system were determined from several field vibration measurements results and were applied to the theoretical mechanical model of the system. The resulting stresses calculated from the Finite Element Model were then compared to the allowable fatigue limits for the piping and valves materials and a life expectancy was calculated. Several different methods were used to this effect.
The package consists of a small natural gas compressor, scrubber, cooler and connecting pipe. The compressor utilizes one bank of cylinders of a 460 in3 Ford V8 engine retrofitted with pistons to compress gas while the other cylinder bank is used to provide power to the unit. The system assembly uses many NPT threaded fittings to join fuel gas lines, valves, process gas pipes and vessels. BP Canada Energy Co. requested AP Dynamics Inc. to investigate and evaluate the performance and life expectancy of the threaded connections on these modularized compressor packages.
